1. Field of the Invention
The present invention relates to a light emitting device and a method of controlling the same. More particularly, the present invention relates to a light emitting device and a method of controlling the same, which can optimize a drive voltage to prevent a stress to peripheral devices, without distorting an optical output of a light emitting unit, and thus improve the efficiency of a system.
2. Description of the Related Art
Conventional display devices include direct view cathode ray tubes (CRT), a flat panel displays (FPD) and front and rear projectors. Exemplary FPDs include a liquid crystal display (LCD) panel and a plasma display panel (PDP). New display technologies, such as an organic electroluminescent (EL), liquid crystal on silicon (LCOS)and a digital light processing (DLP), are continuing to be developed for use in one or more types of display devices.
A display device using LCD, LCOS or a DLP technologies employs a light emitting device, such as a light emitting diode (LED), as a light source. An LED is a point light source, and has a high luminance and good color reproducibility. An LED driven by an electric current minimizes a ripple component of an output electric current so as to improve the quality of the displayed image. Further, an LED driven by an electric current requires a drive unit having a quick response characteristic in view of the characteristics of the display device. To achieve this, a linear current source may be used.
FIG. 1 is a circuit diagram illustrating a conventional LED driving device.
A conventional LED driving device 10 includes a variable voltage source 12, a control logic unit 14, a low-pass filter 16, a transistor 18, a current control unit 20, and a light emitting unit 22.
The variable voltage source 12 generates an optimum voltage so as to improve the efficiency of the LED driving device 10 when the light emitting unit 22 is driven. The control logic unit 14 monitors the voltage of Vd (i.e., Vo-VAK) so as to control the output of the variable voltage source 12, and generates a PWM signal so as to generate a reference voltage to be applied to the light emitting unit 22 using the monitored voltage. The low-pass filter 16 performs smoothing of the PWM signal generated by the control logic unit 14. The transistor 18 is connected in series with the light emitting unit 22, and generates the constant current required in the LED driving device 10 using the voltage provided from the variable voltage source 12. The current control unit 20 adjusts the amount of the current generated by the transistor 18. The light emitting unit 22 includes at least one LED which receives the constant current from the transistor 18 to emit light.
FIG. 2 is a graph depicting a variable output voltage outputted from the variable voltage source and a waveform of an electric current applied to the light emitting unit, according to the conventional LED driving device.
The LED driving device 10 must generate the optimum voltage so that the light emitting unit 22 emits light. In the LED driving device 10, the control logic unit 14 generates the PWM signal so as to output a constant voltage during an early driving stage. The control logic unit 14 controls the optimum value of the output voltage in such a manner that it waits until a time point t1 at which time the variable voltage source 12 has generated a stable initial voltage, and it progressively decreases the pulse width of the PWM signal after the time point t1 to reduce the output voltage.
The time point where the light emitting unit 22 emits light is a point after the time point t1 where the initial voltage is set. From this time point on the control logic unit 14 generates a current command value, so that the current control unit 20 operates. As such, the LED driving device monitors the voltage Vd applied to the transistor at regular intervals during the emission time of the light emitting unit 22, and reduces the pulse width of the PWM signal if the voltage Vd is higher than a predetermined threshold value Vth, while the LED driving device increases the pulse width of the PWM signal if the voltage Vd is lower than the predetermined threshold value Vth, thereby minimizing a thermal loss of the transistor 18 and adjusting the voltage so that the voltage does not affect the light emitting unit 22.
The display device using the above LED light source varies the command value of the output current depending upon brightness information of the image signal to be displayed. Under this condition, it is necessary to vary the voltage, which is applied to the light emitting unit 22 in accordance with the variation of the output current, depending upon the brightness change of the image signal, so that the optical output is not distorted. That is, the output voltage must be quickly varied from a low value to a high value when a dark image is switched over to a bright image. In this case, if the switching speed is low, the light emitting unit 22 may not produce a sufficient amount of luminance. By contrast, the output voltage must be varied from a high value to a low value when a bright image is switched over to a dark image. In this case, if the switching speed is low, the corresponding high voltage is applied to the peripheral devices, and this causes the occurrence of a thermal loss. Consequently, the efficiency of the display device is reduced, and thus a heat radiating structure must be designed correspondingly.
Accordingly, there is a need for an improved a light emitting device and a method of controlling the same, which can optimize a drive voltage to prevent a stress to peripheral devices and thus improve an efficiency of a system, without distorting an optical output of a light emitting unit.